Process for the recovery of ammonium sulphate from it aqueous solutions contaminated by organic substances

ABSTRACT

A process is disclosed for the recovery of ammonium sulphate from its solutions contaminated by organic substances, wherein the ammonium sulphate solution containing the organic impurities is treated by the addition of nitric acid in such a quantity that its concentration in the mother solution of the subsequent evaporation phase does not exceed 15 percent by weight; the solution thus treated being then subjected to evaporation; and the crystallized ammonium sulphate being subsequently separated from the mother solution and from the organic impurities.

United States Patent [191 Scarfi et al.

Jan. 16, 1973 PROCESS FOR THE RECOVERY OF AMMONIUM SULPHATE FROM ITAQUEOUS SOLUTIONS CONTAMINATED BY ORGANIC SUBSTANCES Inventors: AlbertoScarfi, Emanuele Gugliotta,

both of Siracusa; Aldo Giudice, Lentini Siracusa, all of Italy Assignee:SINCAT Societa Industriale Catanese S. P. A., Palermo, Italy Filed: Feb.23, 1971 Appl. No.: 118,176

Foreign Application Priority Data Feb. 27, 1970 ltaly ..2l252 A/7Q US.Cl. ..423/545 Int. Cl ..C0lc 1/24 Field of Search ..23/l20, 119; 423/545References Cited UNlTED STATES PATENTS 9/1969 Miller at al. ..23/l 19FOREIGN PATENTS OR APPLlCATlONS [63,l62 5/l92l Great Britain ..23/l20183,089 7/l922 Great Britain t t t ..23/l20 l,206,505 9/1970 GreatBritain ..23/l20 [86,990 ll/l966 U.S.S.R......... ..23/l20 239,2797/1969 U.S.S.Rr ..23/l20 Primary ExaminerEdward Stern Attorney-Stevens,Davis, Miller & Mosher [57] ABSTRACT 8 Claims, No Drawings PROCESS FORTHE RECOVERY OF AMMONIUM SULPHATE FROM IT AQUEOUS SOLUTIONS CONTAMINATEDBY ORGANIC SUBSTANCES The present invention relates to a process for therecovery of ammonium sulphate from its aqueous solutions contaminated byorganic substances. More particularly, the present invention relates toa process for the recovery of ammonium sulphate resulting from theproduction of acrylonitrile and methacrylonitrile; and still moreparticularly acrylonitrile obtained by synthesisfrom P pylene am2PB39dsl: M

The main impurities in the solutions resulting from; the said lastproduction are generally organic impurities including acrylonitrile,polyacrylonitrile, formamide, acetonitrile, hydrocyanic acid and itspolymers, acrylic acid, and acrylamides.

It is well known that upon evaporation of such solutions there is notonly the drawback of the deposition of tarry substances on the equipmentand especially on the heat exchanging surfaces, but also the recoveredammonium sulphate remains polluted by these substances.

Thus one object of this invention is that of providing a simple andcheap method for recovering ammonium sulphate of a high degree ofpurity, while at the same time avoiding clogging the recovery equipment.

Another object of this invention is that of providing a recovery methodthat will ensure a satisfactory yield of the ammonium sulphate.

These and still other objects are attained by a process that consists inadding to the ammonium sulphate solution, containing the organicimpurities, nitric acid in such a quantity that its concentration in themother solution of the subsequent phase does not exceed percent byweight, in then subjecting the solution thus treated to vaporization,and in then separating the crystallized ammonium sulphate from themother liquor and from the organic impurities.

In fact, it has surprisingly been found in accordance with the presentinvention that the presence of nitric acid during the concentrationoperations brings about the formation of easily separable carbonaceousagglomerates, in as much as these agglomerates show no tendency toadhere to the ammonium sulphate crystals and to the evaporator walls.

The described beneficial action of the HNO manifests itself withexcellent results at relatively low temperatures, for instance at aboutC, as well as at relatively high temperatures, such as for instance atabout 90C or at about 120C (the boiling temperature of the saturatedsolutions of ammonium sulphate containing 15 percent of HNO as well asat intermediate temperatures.

It has proved to be particularly convenient to work at a temperaturebetween and C, that is at the temperatures that are normally developedin the standard vacuum evaporators used in the crystallization ofammonium sulphate.

There takes place simultaneously in these evaporators theinsolubilization of the organic substances as well as thecrystallization of the ammonium sulphate.

More particularly, the solution from which the ammonium sulphate is tobe recovered, and in which the concentration in (NHQ SO may vary withinwide limits such as up to the saturation concentration (there If thesolution intended for the recovery of (NH4)2 S0 turns out to be acidfrom the start (in general this will be an acidity due to sulphuricacid) before the addition of HNO one will neutralize this initialacidity by means of NH until a pH between 5 and 6 is reached, butpreferably a pH of around 5.5. The ammonia may also be introduceddirectly during the vaporization stage of the process. Due to the highsolubility of the resulting nitrate, there is no d anger of itscontaminating the crystallized ammonium sulphate.

The solution thus treated with nitric acid is then subjected toevaporation, for instance in a continuous way under vacuum at 65C.During the evaporation, the organic substances precipitate in the formof carbonaceous particles of just a few tenths of a millimeter in size,and these are uniformly diffused throughout the solution.

The crystallized ammonium sulphate is then separated from the solutionand from the organic impurities by per se known methods such as forinstance by centrifuging.

The ammonium sulphate thus separated is successively washed in the samecentrifuge with water in quantities between 5 and 20 percent (preferablyabout 10 percent) with respect to the salt, which is then suitablydried, for instance, at 90C.

The yield of ammonium sulphate turns out to be greater than 90 percentwhen the content in organic impurities does not exceed 0.2 percent(always expressed as carbon).

The mother liquor and the wash waters are recycled back into theconcentration phase. When the content of organic substances in the(NI-[Q solution at the inlet is quite high (greater than 0.6 percent ofC) it will be necessary, before re-cycling, to subject the recycle waterto filtration in order to free it of the organic impurities insuspension.

Due to this recycling, the concentration of the organic substances inthe mother liquor tends to increase. Since in general it is notconvenient to permit this concentration, under operational conditions,to exceed the value of about 2 percent in order to hinder or prevent anexcessive accumulation of impurities, part of the mother solution mustbe discharged from the circuit. In general there are discharged from 3percent to 30 percent, but preferably from 5 to 10 percent, of themother liquor, expressed as (NH SO participating in the processing.

In the case of solutions having at the inlet a high content in organicsubstances, for instance a content between 0.6 and [.0 percent, it wouldbecome necessary to carry through a considerable discharge of mothersolution, with a consequential lowering of the yield in ammoniumsulphate. In order to maintain the yield and at the same time .to obtaina fairly well purified ammonium sulphate, it is preferred to maintainthe content in carbon in the mother liquor at a higher value (up toabout 2.5 3 percent), and to purify the crystallized (NHJ SO not bysimply washing it in a centrifuge but by double centrifugation, mixingthe salt coming from the first centrifugation with small quantities ofwater. The ammonium sulphate coming from the first centrifugation canalso be kneaded with water and thereafter subjected to a secondcentrifuging. Since the organic substances contained in the mothersolution are partly dissolved and partly undissolved in the form ofcarbonaceous agglomerates, from the intermediate mixture of salt andwater between the first and the second centrifugation it is alsopossible to remove the undissolved organic substances from the salt bydeslim- In order still better to illustrate the invention, someillustrative non-limiting examples are given:

EXAMPLE 1 1n the following test all quantities, for the sake ofconvenience, are referred to 100 g of solution of (NI-102504 whichenters the cycle. All percentage values are percentages by weight.

A solution containing 30 percent of (NI- 80., and organic substances inquantities equal to 0.2 percent, expressed as carbon, and coming fromthe production of acrylonitrile from propylene, ammonia and air, wasevaporated by a continuous process, under vacuum at 60-65C, in anindirect heat-exchanging apparatus.

To 100.000 g of this solution were then added 0.156 g of l-lNOcalculated as 100 percent. The quantity of HNO added to the solution issuch that, under operational conditions, there was produced in theevaporated mother solution a concentration of l-lNO equal to 3 percentwhile the concentration of the organic substances rose to 1.6 percent.

Thereupon there were evaporated 70.1 13 g of water containing underoperational conditions 0.112 g of organic substances expressed as carbonand 0.001 g of HNO The crystallized salt was then centrifuged and washedin the centrifuge with 2.767 g of water, whereupon the wash waters werere-cycled. From the circuit were discharged 5.155 g of mother solutioncontaining, under operational conditions, 2.350 g of (Ni-10 80 0.083 gof organic substances expressed as carbon, anclv 0.155 g ofHNO Afterdrying at 90C, there were obtained 27.655 g of (NHQ SO of a white colorand with -21 percent nitrogen, containing 0.005 g of organic substancesexpressed as carbon (0.018 percent ofC).

The yield of(NH,) SO 92.17 percent.

EXAMPLE 2 A solution containing 34 percent of (NHJ SQ, and organicsubstances in quantities equal to 0.2 percent expressed as carbon, andcoming from the same production as that mentioned in Example 1, wasevaporated under vacuum at 60 65C by a continuous process.

To 100.000 g of this solution were then added 0.168 g of HNO and 0.015 gof NH In this way, there was established in the mother solution, underoperational conditions, a concentration in HNO of 3 percent, of

which one third appeared in the form of NH NO while the concentration ofthe organic substances rose to 1.6

percent.

Thereafter there were evaporated 66.294 g of water containing, underoperational conditions, 0.106 g of carbon and 0.001 g of HNO Thecrystallized salt was centrifuged and then washed in the centrifuge with3.147 g of water and the .wash waters were then re-cycled. There weredischarged from the circuit 5.558 g of mother solution containing, underoperational conditions, 2.527 g of (NI-10 80,, 0.089 g of carbon, 0.1 11g of HNO; and 0.071 g of NH NO After drying at C, there were obtained31.478 g of (NH,),SO of a white color and with 20 21 percent ofnitrogen, and containing 0.005 g of carbon (0.016 g of carbon).

The yield of (NH SO 92.57 percent.

EXAMPLE 3 A solution containing 30 percent of (N1-1 SO and organicsubstances in quantities equal to 0.2 percent, expressed as carbon, andcoming from the same production as that mentioned in the precedingexamples, was evaporated by a continuous process under vacuum at 60 65C.

To 100.000 g of this solution were then added 0.514 g of 1-1'NO Thissolution was then evaporated in such a way as to bring about in themother solution, under operational conditions, a concentration of HNOequal to 10 percent, while the concentration in organic substances roseto 1.6 percent.

70.599 g of water then evaporated which, under operational conditions,contained 0.113 g of carbon and 0.003 g of HNO The crystallized salt wasthen centrifuged and successively washed in the centrifuge with 2.756 gof water, after which the wash waters were recycled.

From the circuit were discharged 5.106 g of mother solution containing,under operational conditions, 2.440 g of (N1-1 SO 0.082 g of carbon and0.511 g of HNO After drying at 90C, there were obtained 27.565 g of awhite (NH SO. having 20-21 percent of nitrogen, and containing 0.005 gof carbon (0.018 percent of carbon).

The yield in (NH,) SO was equal to 91.87 percent.

EXAMPLE 4 tional conditions, a concentration of l-lNO equal to 3 1percent, while the concentration of the organic substances grew to 1.9percent.

There were evaporated 67.219 g of watercontaining, under operationalconditions, 0.148 g of carbon and 0.001 g of HNO and the crystallizedsalt was centrifuged and then washed in the centrifuge with 2.520 g ofwater. The wash waters were re-cycled to the evaporation stage, and fromthe circuit were discharged 10.352 g of mother solution containing,

under operational conditions, 4.744 g of (NH SO 0.197 g of carbon and0.311 g of HNO and, after drying, there were obtained 25.261 g of (Ni-80 of a white color and having 20 21 percent of nitrogen, and containing0.005 g of carbon (0.020 percent of carbon).

The yield of(N1-l,,) SO amounted to 84.19 percent.

EXAMPLE 5 A solution containing 27 percent of (NH SO and organicsubstances in quantities equal to 0.6 percent, expressed as carbon, andcoming from the same production as that indicated in the precedingexamples, was evaporated in a continuous process under vacuum at 60*65C.

To 100.000 g of this solution were then added 0.623 g of HNO so as toobtain, under operational conditions, in the mother solution aconcentration of l-lNO equal to 5 percent, while the concentration ofthe organic substances rose to 2.7 percent.

There were evaporated 70.079 g of water containing, under operationalconditions, 0.259 g of carbon and 0.002 g of HNO and the crystallizedsalt was then centrifuged and successively washed in the centrifuge with3.200 g of water. The wash waters were then re-cycled back to theevaporation stage and from the circuit were discharged 12.408 g ofmother solution containing, under operational conditions, 5.670 g of (NHSO 0.335 g of carbon and 0.621 g of HNO After drying there were obtained21.336 g of (NH ),SO of a white color and having 20-21 percent ofnitrogen, and containing 0.006 g of carbon (0.028 percent of carbon).

The yield of (NI-10 80., amounted to 79.00 percent.

The nitric acid chemical action has not been found out so far; anyhow ithas been ascertained to be of the utmost importance that theconcentration takes place in acid environment due to nitric acid; thesimple heat treatment of the solution with l-lNO does not cause anyeffect: in fact, when the solution is hot-treated with HNO andneutralized with Nl-l before concentrating it, the organic impuritiespollute both the ammonium sulphate and the apparatuses.

What is claimed is:

1. A process for the recovery of ammonium sulphate from its solutionscontaminated by organic substances,

wherein the ammonium sulphate solution containing the organic impuritiesis treated by the addition of nitric acid in such a quantity that itsconcentration in the mother solution of the subsequent evaporation phasedoes not exceed 15% by weight; the solution thus treated is subjected toevaporation at temperatures ranging between 40 and C; and thecrystallized ammonium sulphate is subsequently separated from the mothersolution and from the organic impurities which precipitate in the formof carbonaceous agglomerates having no tendency to adhere to theammonium sulphate crystals and to the equipment.

2. A process according to claim 1, wherein the concentration in l-lNO ofthe mother solution has a value between 0.5 percent and 15 percent byweight.

3. A process according to claim 1, wherein the concentration in HNO ofthe mother solution has a value between 1 percent and 5 percent byweight.

4. A process according to claim 1, wherein the crystallized ammoniumsulphate is separated from the mother solution and from the organicimpurities by centrifuging.

5. A process according to claim 1, wherein, in the case of (NH SOsolutions having at the inlet a content in organic substances greaterthan 0.6 percent by weight expressed as carbon, the mother liquor comingfrom the centrifugation as well as the wash waters are subjected tofiltration before recycling into the concentration stage, in order topurify them from the organic impurities present in them in suspension.

6. A process according to claim 1, wherein, in the case of (NH.,) SOsolutions having at the inlet a content of organic substances greaterthan 0.6 percent by weight expressed as carbon, the ammonium sulphate issubjected to a double centrifugation and between the first and thesecond centrifugation it is kneaded with water.

7. A process according to claim 1, wherein, in the case in which theammonium sulphate solution at the inlet is an acid solution, the acidityis first neutralized with NFL, to a pH of from 5 to 6.

8. A method according to claim 1, wherein the recycling mother liquor isdischarged from the circuit in quantities ranging between 3 and 30percent, expressed as (NH SO participating in the processing.

2. A process according to claim 1, wherein the concentration in HNO3 ofthe mother solution has a value between 0.5 percent and 15 percent byweight.
 3. A process according to claim 1, wherein the concentration inHNO3 of the mother solution has a value between 1 percent and 5 percentby weight.
 4. A process according to claim 1, wherein the crystallizedammonium sulphate is separated from the mother solution and from theorganic impurities by centrifuging.
 5. A process according to claim 1,wherein, in the case of (NH4)2SO4 solutions having at the inlet acontent in organic substances greater than 0.6 percent by weightexpressed as carbon, the mother liquor coming from the centrifugation aswell as the wash waters are subjected to filtration before recyclinginto the concentration stage, in order to purify them from the organicimpurities present in them in suspension.
 6. A process according toclaim 1, wherein, in the case of (NH4)2SO4 solutions having at the inleta content of organic substances greater than 0.6 percent by weightexpressed as carbon, the ammonium sulphate is subjected to a doublecentrifugation and between the first and the second centrifugation it iskneaded with water.
 7. A process according to claim 1, wherein, in thecase in which the ammonium sulphate solution at the inlet is an acidsolution, the acidity is first neutralized with NH3 to a pH of from 5 to6.
 8. A method according to claim 1, wherein the recycling mother liquoris discharged from the circuit in quantities ranging between 3 and 30percent, expressed as (NH4)2SO4, participating in the processing.